The frequency depends on the MLF techniques being used. At the beginning, it is recommended to do an analysis immediately following alcoholic fermentation (or even earlier in the event of co-inoculation) and then to check batches every one to two weeks.
Different methods can be used and will depend upon whether or not the technique is to be carried out internally and the degree of precision required.
When a precise measurement is needed, enzymatic determination is recommended. Otherwise monitoring via FTIR measurements or paper chromatography may be sufficient.
A legal threshold limit for malic acid in wines is defined. This level varies depending on appellations and is generally between 0.3 and 0.4 g/L. In practice, malolactic fermentation is considered to be complete when the malic acid content is less than 0.2 g/L.
Monitoring acetic acid (the main component of volatile acidity) is equally important as it is produced naturally during MLF. It also indicates a microbiological deviation in the event of a problem.
Similarly, residual sugars should be monitored if any remain. Although it is preferable that residual sugars be consumed before MLF, if there are any left at the end of MLF, they should be monitored and their complete breaking down verified before sulphiting. Note that light sulphiting (0.5 g/hL) does not have an adverse effect on the yeast but slows down the bacteria.